Add support for stdout/stderr/stdin streams

[Trolldemorted]

Thanks for providing an simple abstraction over russh!

We are often running commands that take minutes/hours to complete, and getting some feedback before the command has finished would greatly increase the user experience. If execute (or a cousin function) could expose stdout/err like stdlib's/tokio's Command, we could consume stdout/err and (and maybe push stdin?) while the command runs.

[Miyoshi-Ryota]

Thank you for the great suggestion. I'm very interested in it. I don't have time to implement it right now, but I'll definitely merge it if a pull request comes in.